lilnickers, I HAVE seen something very similar to this. Back in the early 60s, we had a mini mare (aka midget Shetland) that had a foal. She had been in foal when my dad bought her. She had the cutest little bay pinto..just like herself..filly. The filly had a bump like that one , only bigger. It was about 3/4" -1" or so that stuck out, and about 1" diameter. It didn't "leak" that I can remember, but it was a bit soft, and would go in and out. When I say "in and out" I mean that sometimes it would be out, and sometimes it would be sunk in for a few days then come back out.
She also had a curly tail..not the hair, the dock grew in a circle and gave the appearance that she had a docked tail.
We had our good horse vet look at it, and he said he had never seen it in a horse before, but had seen it in pigs. He said that it was caused by a lot of inbreeding in hogs. This mare had come from a pony ride breeder, and so she probably WAS very inbred.
We had called the vet because the bump went in, and flys got in the hollow and she had maggots in there (so maybe it did leak a bit and that is what drew the flys). So, the next time it came out, he cut it off and it looked ok then.
I have no idea what happened to the pair of them as we took them to auctiion. The fact of the filly having this problem, plus the mare had colic several times a week, led my dad to say they had to go.
edited to add that I always said that she was a unicorn